One piece of fruit is either one whole fruit like an apple, orange, pear, or banana, or 6 oz. of fruit (such as pineapple, berries, or melon). If the size of the fruit is unusual (for example, a very tiny apple), or the certainty of using the scale provides more mental freedom, you can choose to weigh 6 oz. of fruit. But one piece is fine too. Eating one apple or one pear, even though they tend to weigh more than 6 oz., is fine; don’t worry about it. The “piece of fruit” is the serving size.
Some people are triggered when they don’t weigh their fruit, so they choose to routinely weigh it.